1. Field of the Invention
The present invention relates to a ball joint of, for example, the steering or suspension system of a vehicle. More particularly, the invention relates to a ball joint having a single-piece ball seat of hard plastic.
2. Description of the Prior Art
A ball joint normally consists of a ball socket, in which a ball seat is fitted, and a ball stud with its ball portion slidably inserted in the ball seat. A ball joint, used in a vehicle's suspension system, must have a ball that slides smoothly in its ball seat, have high rigidity and high load durability. To meet these requirements, hard plastic is frequently used for the ball seat. However, if there is a slight irregularity in the inner diameter of the ball socket, a ball seat made of hard plastic tends to fracture if excessive friction exists between the ball seat and the ball. However, some friction is essential for the ball joint to operate smoothly. Since an appropriate amount of tolerable friction is not easily determined, the inner diameter of the socket is conventionally made to a very strict tolerance, resulting in high manufacturing costs.
In an attempt to alleviate this problem, Japanese Patent Publication Laid-Open No. 45161/1975 describes a ball seat with an upper inner surface having a more steeply inclined inner curvature than the spherical surface of the ball installed therein. This configuration utilizes the reaction force created by upward thrust on the upper inner surface of the ball seat as preload to the ball portion at the time of assembly of the ball portion of the ball stud. The ball seat, with slits to provide elasticity, is installed prior to assembly of the ball portion and holds the ball portion therein. This ball joint has a space between the ball seat and an end member (cover) above the ball seat, preventing the ball seat from touching the cover, thus providing space for expansion.
Essentially, the ball joint described in the above patent publication applies preload from the ball seat to the ball portion. However, the slits in the ball seat alone are not sufficient to ensure adequate elasticity to obtain a proper degree of stable friction. Moreover, since the ball seat has slits, the available load bearing area is proportionately reduced. Reducing the load bearing area in this manner is a disadvantage when compared to other ball joints with the same ball size because it may require using a larger ball joint to compensate for the reduced load bearing area.
In summary, conventional ball joints have problems, i.e., the inner diameter of the socket must be manufactured to a very close tolerance to prevent fracturing of the hard plastic ball seat and/or the ball joint has poor load durability.